1. Field of the Invention
This invention generally relates to optical multimeters and more particularly to signal generating portions thereof.
2. Description of the Related Art
The telecommunications network serving the United States and the rest of the world is presently evolving from analog to digital transmission with ever increasing bandwidth requirements. Fiber optic cable has proved to be a valuable tool, replacing copper cable in nearly every application from large trunks to subscriber distribution plants. Fiber optic cable is capable of carrying much more information than copper with lower attenuation.
The T-1 standards committee ANSI has provided a draft document, xe2x80x9cANSI T1.105-1988xe2x80x9d, dated Mar. 10, 1988, which sets forth specifications for rate and format of signals which are to be used in optical interfaces. The provided specifications detail the Synchronous Optical Network (SONET) standard. SONET defines a hierarchy of multiplexing levels and standard protocols which allow efficient use of the wide bandwidth of fiber optic cable, while providing a means to merge lower level DS0 and DS1 signals into a common medium. In essence, SONET established a uniform standardization transmission and signaling scheme, which provided a synchronous transmission format that is compatible with all current and anticipated signal hierarchies. Because of the nature of fiber optics, expansion of bandwidth is easily accomplished.
Currently this expansion of bandwidth is being accomplished by what is known as xe2x80x9cwavelength division multiplexingxe2x80x9d (WDM), in which separate subscriber/data sessions may be handled concurrently on a single optic fiber by means of modulation of each of those subscriber datastreams on different portions of the light spectrum. WDM is therefore the optical equivalent of frequency division multiplexing (FDM). Current implementations of WDM involve as many as 128 semiconductor lasers each lasing at a specific center frequency within the range of 1525-1575 nm. Each subscriber datastream is optically modulated onto the output beam of a corresponding semiconductor laser. The modulated information from each of the semiconductor lasers is combined onto a single optic fiber for transmission. The data structure of a basic SONET signal at a typical data rate of 51.84 Mbps, a.k.a. an STS-1 signal, has 9 rows of 90 columns of 8 bit bytes at 125 xcexcs frame period. The first three columns of bytes in the SONET signal are termed the transport overhead (TOH) bytes that are used for various control purposes. The remaining 87 columns of bytes constitute the STS-1 synchronous payload envelope (SPE). As this digital signal is passed across a SONET network, it will be subject at various intervals to amplification by, for example, Erbium doped amplifiers and coherency correction by, for example, optical circulators with coupled Bragg filters. At each node in the network, e.g. central office or remote terminal, optical transceivers mounted on fiber line cards are provided. On the transmit side, a framer permits SONET framing, pointer generation and scrambling for transmission of data from a bank of lasers and associated drivers, with each laser radiating at a different wavelength. On the receive side, the incoming signals are detected by photodetectors separated into channels, framed and decoded.
As more and more optical signal equipment (transmitting, receiving, amplification, coherence and switching) is being designed and utilized, a need has arisen for optical multimeters, e.g. signal generators and detectors, which can be used to test the various components of an optical, e.g. SONET, network. What is needed is a tunable optical signal generator that does not require the complex control systems relied on by prior art devices. Those control systems utilize closed loop feedback of wavelength or position to select the output wavelength of the optical signal generator. As a result they are expensive and exhibit a large form factor.
The present invention provides an optical multimeter for use in calibrating and testing optical components. The optical multimeter includes an optical analog/digital signal generator for delivering an output beam over a wide range of frequencies, duty cycles and amplitudes. The optical signal generator includes a tunable laser. The tunable laser exhibits a small form factor, due in part to a novel wavelength control process which utilizes an open loop system to maintain precise output wavelength control, without the requirement of either a wavelength or position feedback device.
In still another embodiment of the invention, a wavelength control system for a tunable laser is disclosed. The wavelength control system includes a lookup table, a start indicator, and a first logic. The lookup table stores values correlating output wavelength and drive signals relative to a starting position of the tunable feedback device. The start indicator couples to the tunable feedback device to indicate the starting position of the tunable feedback device. The first logic couples to the start indicator, the tunable feedback device and the lookup table. The first logic generates drive signals to position the tunable feedback device at the starting position as indicated by said start indicator. The first logic responds to a next selected output wavelength to compare the next selected wavelength to the values in the lookup table and generates corresponding drive signals to tune the tunable feedback device to the next selected wavelength.
In yet another embodiment of the invention a method for controlling output wavelength in a tunable laser is disclosed. The method includes the acts of:
storing values correlating output wavelength and drive signals relative to a starting position of the tunable feedback device;
positioning the tunable feedback device at the starting position;
selecting a next selected output wavelength;
comparing the next selected output wavelength to the values stored in said act of storing;
calculating the drive signals required to re-position the tunable feedback device at the next selected output wavelength responsive to said act of comparing; and
generating the drive signals to tune the tunable feedback device to the next selected wavelength responsive to said act of comparing.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.